1. Field of the Invention
The present invention relates to a cold cathode lamp.
2. Description of the Related Art
A schematic sectional view of a conventional cold-cathode lamp is shown in FIG. 14. The conventional cold-cathode lamp shown in FIG. 14 has internal electrodes 2 and 3 inside a glass tube 1. Part of each of the internal electrodes 2 and 3 leads out of the glass tube 1 through its wall to function as an electrode terminal. In this structure, the interior of the glass tube 1 is air-tight. The interior wall surface of the glass tube 1 is coated with a fluorescent material. Inside the air-tight glass tube 1 are typically sealed neon and argon in a ratio of 95:5 or 80:20, for instance, such that the overall pressure inside the glass tube 1 is in the range of 10.7×103 to 5.3×103 Pa (≈80 to 40 Torr) along with several milligrams of mercury. Instead of mercury, xenon may be sealed in.
When the lamp voltage (the voltage between the internal electrodes) reaches the discharge start voltage VS, electric discharge starts. The electric discharge causes the mercury or xenon to generate ultraviolet rays, which cause the fluorescent material coating the inner wall surface of the glass tube 1 to glow.
In terms of its equivalent circuit, the conventional cold-cathode lamp shown in FIG. 14 acts as a resistor whose resistance decreases nonlinearly as the current through it increases, and exhibits a nonlinear negative impedance characteristic, like the V-I characteristic shown in FIG. 15 (see, for example, Patent Document 3 listed below).
One use of the conventional cold-cathode lamp shown in FIG. 14 is as a backlight in a liquid crystal display device. For a liquid crystal display device with a large display screen, a plurality of cold-cathode lamps are used arranged side by side. In this case, if all the cold-cathode lamps can be driven in parallel, they can all be fed with an equal voltage, requiring a single power supply.
Now we will consider parallel driving of a plurality of (for example, three) cold-cathode lamps. The V-I characteristic of the cold-cathode lamp varies from one individual to another, a first to a third cold-cathode lamp exhibiting different V-I characteristics as represented by the V-I characteristic curves T1 to T3 in FIG. 16. The first to third cold-cathode lamps are fed with an equal alternating-current voltage, which is then raised. When the alternating-current voltage has risen to reach the discharge start voltage VS1 of the first cold-cathode lamp, it is lit; thereafter the nonlinear negative impedance characteristic of the first cold-cathode lamp causes the voltage across it to fall. Since the voltage across the second and third cold-cathode lamps equals that across the first, the alternating-current voltage never reaches the discharge start voltages VS2 and VS3 of the second and third cold-cathode lamps. Thus, when a plurality of cold-cathode lamps are simply driven in parallel, only one of them can be lit. This is the reason that, generally, a plurality of power supply circuits are provided one for each of a plurality of cold-cathode lamps to light them all. Inconveniently, however, not only does this configuration require as many power supply circuits as there are cold-cathode lamps, incurring accordingly high costs, but it also thwarts size reduction, weight reduction, and cost reduction. On the other hand, generally, the cold-cathode lamps are connected to their respective power supply circuits via harnesses (also called leads) and connectors. This makes the fitting of cold-cathode lamps troublesome, resulting in low efficiency of the assembly of illumination devices and the like employing cold-cathode lamps, and also makes the removal of cold-cathode lamps troublesome, resulting in low efficiency of the replacement of cold-cathode lamps, and of the disassembly of illumination devices and the like employing cold-cathode lamps.
As a solution to the inconveniences mentioned above, there have been developed external-electrode fluorescent lamps (see, for example, Patent Documents 1 and 2 listed below). A schematic sectional view of an external-electrode fluorescent lamp is shown in FIG. 17. In FIG. 17, such parts as find their counterparts in FIG. 14 are identified by common reference signs and their detailed description will not be repeated. In the external-electrode fluorescent lamp shown in FIG. 17, as compared with the conventional cold-cathode lamp shown in FIG. 14, the internal electrodes 2 and 3 are omitted, and instead external electrodes 4 and 5 are formed one at each end of the glass tube 1. In this structure, the interior of the glass tube 1 is air-tight.
In the external-electrode fluorescent lamp shown in FIG. 17, when the lamp voltage (the voltage between the external electrodes) reaches the discharge start voltage VS', electric discharge starts. The electric discharge causes the mercury or xenon to generate ultraviolet rays, which cause the fluorescent material coating the inner wall surface of the glass tube 1 to glow.
The interior of the glass tube 1 has a nonlinear negative impedance characteristic, and is insulated by the glass from the external electrodes 4 and 5. Thus, in terms of its equivalent circuit, the external-electrode fluorescent lamp shown in FIG. 17 acts as a serial circuit composed of a resistor whose resistance decreases nonlinearly as the current through it increases and capacitors connected one to each terminal of the resistor. Accordingly, the external-electrode fluorescent lamp shown in FIG. 17 as a whole exhibits a nonlinear positive impedance characteristic, like the V-I characteristic shown in FIG. 18.
Now we will consider parallel driving of a plurality of (for example, three) external-electrode fluorescent lamps. The V-I characteristic of the external-electrode fluorescent lamp varies from one individual to another, a first to a third external-electrode fluorescent lamps exhibiting different V-I characteristics as represented by the V-I characteristic curves T1′ to T3′ in FIG. 19. The first to third external-electrode fluorescent lamps are fed with an equal alternating-current voltage, which is then raised. When the alternating-current voltage has risen to reach the discharge start voltage VS1′ of the first external-electrode fluorescent lamp, it is lit. Thereafter, as the output of the power supply increases, the alternating-current voltage rises. When the alternating-current voltage reaches the discharge start voltage VS2′ of the second external-electrode fluorescent lamp, it is lit and, when the alternating-current voltage reaches the discharge start voltage VS3′ of the third external-electrode fluorescent lamp, it is lit. Thus, when a plurality of external-electrode fluorescent lamps are simply driven in parallel, they can all be lit.
On the other hand, thanks to the external electrodes provided on the exterior surface of the glass tube, in illumination devices and the like employing external-electrode fluorescent lamps, these can be held by the resilience of holding members formed of a resilient metal material (for example, spring steel), with the holding members pinching in them the external electrodes of the external-electrode fluorescent lamps. This permits the external-electrode fluorescent lamps to be supplied with electric power via the holding members. Conveniently, this structure makes the fitting and removal of external-electrode fluorescent lamps easy.
Patent Document 1: JP-A-2004-31338
Patent Document 2: JP-A-2004-39264
Patent Document 3: JP-A-H7-220888 (FIG. 4)
Patent Document 4: JP-A-2004-39336
Patent Document 5: JP-A-H5-121049
Patent Document 6: JP-A-S64-82452
Patent Document 7: JP-A-2003-100482
Patent Document 8: JP-A-H11-40109
Patent Document 9: JP-U-H2-41362
Patent Document 10: JP-A-H6-84499
Inconveniently, however, in an external-electrode fluorescent lamp, since the glass between the external electrodes and the interior space of the glass tube acts as the dielectric between the electrodes of the capacitor as one component of the equivalent circuit of the external-electrode fluorescent lamp, charged particles collide with the part of the inner wall surface of the glass tube facing away from the external electrodes, wearing (sputtering) off the inner wall surface of the glass tube locally. As the inner wall surface of the glass tube wears off, the capacitance of its worn part grows, causing charged particles collide with that part in an increasingly concentrated fashion, eventually forming pinholes and making it impossible to keep the interior of the glass tube air-tight. Thus external-electrode fluorescent lamps are unsatisfactorily reliable.